Ordnance: Polsten 20mm Cannon
Developed by Polish engineers in the late 1930s the Polsten became a cheaper and simpler alternative to the ubiquitous 20mm Oerlikon. The Poles managed to complete a prototype in 1939 just before Germany and the USSR invaded. They smuggled the prototype and designs out of occupied Poland to Britain where Polish, Czech and British engineers at the Royal Small Arms Factory at Enfield completed the development of the weapon.
The Polsten was simpler and easier to manufacture with 119 parts compared to the Oerlikon’s 250. This also meant it was significantly cheaper to produce costing approximately £60 to £70 rather than the £350 it cost to manufacture an Oerlikon. Despite this huge saving the Polsten was a lighter weapon with slightly faster muzzle velocity and a higher effective ceiling with projectiles reaching up to 2,200m (7,200ft).

Diagrams of the Polsten from a 1944 manual showing the 30-round box magazine and 60-round drum magazine (source)
The Polsten was fully automatic, firing a 20mm high explosive shell which weighed 119g. The weapon fed from either a 30-round box magazine, which had two double stacks funneling into a single feed, or 60-round drum magazines commonly used by the Oerlikon. The Polsten did not have a locked breech and used an API blowback action rather than short recoil. It could achieve rates of fire up to 450 rounds per minute.
Britain manufactured both the Polsten and Oerlikon during the war with the John Inglis factory in Canada also producing Polstens. The guns were mounted in a variety of vehicles in a number of different mounts. A simple wheeled carriage for a single cannon was initially used which could be pulled by jeeps, trucks or horses. The Allies also developed double, triple (see image #2) and quad mounts to provide greater firepower. The British built over 100 British Centaur, AA tanks with dual cannon mounts to provide mobile anti-aircraft fire during Operation Overlord.

Centaur Anti-Aircraft Tank with two Polsten 20mm cannons (source)
British and Commonwealth forces deployed the Polsten across Europe seeing action in France, Holland, Italy and Germany. During Operation Market Garden gliders ferried single cannons for use by paratroops. During the D-Day landings the Royal Armoured Corps was equipped with specially built Centaur AA tanks mounted with Polstens. Trucks with quad mounts provided Anti-Aircraft cover for troops advancing into Germany. Polstens were also mounted on board naval vessels (see image #4) and saw use in the Far East.

Centurion MkI tank with an independently mounted Polsten in the turret (source)
While excellent AA guns Polstens could also be thrust into ground roles. At Arnhem the 1st Airborne Reconnaissance Squadron had two jeep-pulled Polstens which they used against German infantry and vehicles (see image #3). The Polsten remained in British service until the late 1950s. The Centurion MkI tank had a Polsten cannon mounted in its turret. This was later replaced by a medium machine gun because the 20mm shell was thought to be unnecessarily large for use against ground troops.
The Polsten proved to be a highly effective weapon during the war and the British were lucky its Polish designers were able to smuggle it out of occupied Poland.
Sources:
Twentieth Century Artillery, I. Hogg (2000)
Handbook of the Gun, Machine, Polsten, 20mm. MkI, (1944) [source]
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